Converting a bi-predicted merging candidate in a merging candidate list for a prediction unit of a picture

ABSTRACT

Motion compensation requires a significant amount of memory bandwidth, especially for smaller prediction unit sizes. The worst case bandwidth requirements can occur when bi-predicted 4×8 or 8×4 PUs are used. To reduce the memory bandwidth requirements for such smaller PUs, methods are provided for restricting inter-coded PUs of small block sizes to be coded only in a uni-predictive mode, i.e., forward prediction or backward prediction. More specifically, PUs of specified restricted sizes in bi-predicted slices (B slices) are forced to be uni-predicted.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.16/437,098, filed Jun. 11, 2019, which is a continuation of U.S. patentapplication Ser. No. 15/451,085, filed Mar. 6, 2017, now U.S. Pat. No.10,362,326, issued Jul. 23, 2019, which is a continuation of U.S. patentapplication Ser. No. 13/864,159, filed Apr. 16, 2013, now U.S. Pat. No.9,591,312, issued Mar. 7, 2017, which application claims benefit of U.S.Provisional Patent Application Ser. No. 61/625,256, filed Apr. 17, 2012,U.S. Provisional Patent Application Ser. No. 61/625,811, filed Apr. 18,2012, and U.S. Provisional Patent Application Ser. No. 61/651,652, filedMay 25, 2012, all of which are incorporated herein by reference in theirentirety.

BACKGROUND OF THE INVENTION Field of the Invention

Embodiments of the present invention generally relate to memorybandwidth reduction for motion compensation in video coding.

Description of the Related Art

Video compression, i.e., video coding, is an essential enabler fordigital video products as it enables the storage and transmission ofdigital video. In general, video compression techniques applyprediction, transformation, quantization, and entropy coding tosequential blocks of pixels in a video sequence to compress, i.e.,encode, the video sequence. Video decompression techniques generallyperform the inverse of these operations in reverse order to decompress,i.e., decode, a compressed video sequence.

The Joint Collaborative Team on Video Coding (JCT-VC) of ITU-T WP3/16and ISO/IEC JTC 1/SC 29/WG 11 is currently developing thenext-generation video coding standard referred to as High EfficiencyVideo Coding (HEVC). HEVC is expected to provide around 50% improvementin coding efficiency over the current standard, H.264/AVC, as well aslarger resolutions and higher frame rates. To address theserequirements, HEVC utilizes larger block sizes then H.264/AVC. In HEVC,the largest coding unit (LCU) can be up to 64×64 in size, while inH.264/AVC, the macroblock size is fixed at 16×16. Further, HEVC allowsmore variation in the sizes of prediction blocks, e.g., 4×4, 8×4, 4×8,etc.

Motion compensation in H.264/AVC is to known to require a large amountof memory bandwidth in embedded decoder implementations, such as smartphones, set-top boxes, etc. Allocation of 1 Gbyte/second (GB/second) ormore of memory bandwidth to an H.264/AVC high definition (HD) decoder toguarantee proper decoding of worst-case video bit streams is notuncommon. Memory bandwidth for motion compensation is determined byfactors such as prediction block size, the extent of the filter used inmotion compensation, and the access characteristics of the memory. HEVCspecifies an 8-tap filter for interpolation in motion compensation ascompared to the 6-tap filter of H.264/AVC, thus the memory bandwidthrequirements for motion compensation in HEVC are much higher thanH.264/AVC for the same prediction block size. For the worst caseprediction block size of 4×4, the memory bandwidth for motioncompensation in HEVC (i.e., (7+4)*(7+4)*2/(5+4)/(5+4)/2)) is increasedby about 50% over H.264/AVC. HEVC does allow inter-prediction of thesmallest prediction block size (4×4) to be disabled, which helps reducememory bandwidth requirements. If inter-prediction of 4×4 predictionblocks is disabled, motion compensation of 4×8 or 8×4 bi-directionallypredicted prediction blocks becomes the worst case. In this instance,the relative memory bandwidth increase of HEVC as compared to H.264/AVCis approximately 2%, (i.e., (7+4)*(7+8)*2/(5+4)/(5+4)/2/2).

Further, HEVC is designed to support Ultra High Definition (UHD) video,which further increases the memory bandwidth requirements for motioncompensation. The memory bandwidth for motion compensation of HUD videois expected to be an increasing bottleneck. Thus, further reduction ofthe worst cast memory bandwidth requirements for motion compensation isdesirable.

SUMMARY

Embodiments of the present invention relate to methods, apparatus, andcomputer readable media for reducing memory bandwidth need for motioncompensation. In one aspect, a method for processing a prediction unit(PU) of a picture is provided that includes determining that the PU isin a bi-predicted slice of the picture, and restricting prediction ofthe PU to uni-prediction when a size of the PU is a predetermined size.

In one aspect, an apparatus configured for processing a prediction unit(PU) of a picture is provided that includes means for determining thatthe PU is in a bi-predicted slice of the picture, and means forrestricting prediction of the PU to uni-prediction when a size of the PUis a predetermined size.

In one aspect, a non-transitory computer readable medium storingsoftware instructions is provided. The software instructions, whenexecuted by a processor, cause a method for processing a prediction unit(PU) of a picture to be performed. The method includes determining thatthe PU is in a bi-predicted slice of the picture, and restrictingprediction of the PU to uni-prediction when a size of the PU is apredetermined size.

BRIEF DESCRIPTION OF THE DRAWINGS

Particular embodiments will now be described, by way of example only,and with reference to the accompanying drawings:

FIG. 1 is an example illustrating quadtree partitioning of a 64×64largest coding unit (LCU);

FIG. 2 is a block diagram of a digital system;

FIG. 3 is a block diagram of a video encoder;

FIG. 4 is a block diagram of a video decoder;

FIGS. 5 and 6 are flow diagrams of methods;

FIG. 7 is an example; and

FIG. 8 is a block diagram of an illustrative digital system.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

Specific embodiments of the invention will now be described in detailwith reference to the accompanying figures. Like elements in the variousfigures are denoted by like reference numerals for consistency.

As used herein, the term “picture” may refer to a frame or a field of aframe. A frame is a complete image captured during a known timeinterval. For convenience of description, embodiments of the inventionare described herein in reference to HEVC. One of ordinary skill in theart will understand that embodiments of the invention are not limited toHEVC.

In HEVC, a largest coding unit (LCU) is the base unit used forblock-based coding. A picture is divided into non-overlapping LCUs. Thatis, an LCU plays a similar role in coding as the macroblock ofH.264/AVC, but it may be larger, e.g., 32×32, 64×64, etc. An LCU may bepartitioned into coding units (CUs) and CUs may be partitioned intoprediction units (PUs). A CU is a block of pixels within an LCU and theCUs within an LCU may be of different sizes. The partitioning is arecursive quadtree partitioning. The quadtree is split according tovarious criteria until a leaf is reached, which is referred to as thecoding node or coding unit. The maximum hierarchical depth of thequadtree is determined by the size of the largest CU (LCU) and thesmallest CU (SCU) specified for a picture. In recent versions of HEVC,the largest CU size and minimum CU size permitted are 64×64 and 8×8,respectively.

The coding node is the root node of two trees, a prediction tree and atransform tree. A prediction tree specifies the position and size ofprediction units (PU) for a CU. A transform tree specifies the positionand size of transform units (TU) for a CU. A PU may not be larger than aCU, and the size may be, for example, 8×4, 4×8, 8×8, 16×8, 8×18, 16×16,etc. A transform unit may not be larger than a CU and the size of atransform unit may be, for example, 4×4, 8×8, 16×16, and 32×32. Thesizes of the TUs and PUs for a CU are determined by the video encoderduring prediction based on minimization of rate/distortion costs. FIG. 1shows an example of a quadtree based LCU to CU/PU decompositionstructure in which the size of the SCU is 16×16 and the size of the LCUis 64×64.

As used herein, a co-located PU or temporally co-located PU is arectangular or square area in a reference picture having the samecoordinates, size, and shape of a PU in a picture currently beingencoded or decoded, i.e., a PU for which a merging candidate list or anadvanced motion vector predictor (AMVP) candidate list is beingconstructed. As is well-known, PU partitioning may change from LCU toLCU, and from picture to picture. Thus, a co-located PU does notnecessarily correspond to an actual PU of the reference picture. Rather,depending on the size, the co-located PU may overlap one actual PU,multiple actual PUs, portions of several actual PUs, a portion of anactual PU, etc. in the reference picture.

Various versions of HEVC are described in the following documents, whichare incorporated by reference herein: T. Wiegand, et al., “WD3: WorkingDraft 3 of High-Efficiency Video Coding,” JCTVC-E603, JointCollaborative Team on Video Coding (JCT-VC) of ITU-T SG16 WP3 andISO/IEC JTC1/SC29/WG11, Geneva, CH, Mar. 16-23, 2011 (“WD3”), B. Bross,et al., “WD4: Working Draft 4 of High-Efficiency Video Coding,”JCTVC-F803_d6, Joint Collaborative Team on Video Coding (JCT-VC) ofITU-T SG16 WP3 and ISO/IEC JTC1/SC29/WG11, Torino, IT, Jul. 14-22, 2011(“WD4”), B. Bross. et al., “WD5: Working Draft 5 of High-EfficiencyVideo Coding,” JCTVC-G1103_d9, Joint Collaborative Team on Video Coding(JCT-VC) of ITU-T SG16 WP3 and ISO/IEC JTC1/SC29/WG11, Geneva, CH, Nov.21-30, 2011 (“WD5”), B. Bross, et al., “High Efficiency Video Coding(HEVC) Text Specification Draft 6,” JCTVC-H1003, Joint CollaborativeTeam on Video Coding (JCT-VC) of ITU-T SG16 WP3 and ISO/IECJTC1/SC29/WG1, Geneva, CH, Nov. 21-30, 2011 (“HEVC Draft 6”), B. Bross,et al., “High Efficiency Video Coding (HEVC) Text Specification Draft7,” JCTVC-I1003_d0, Joint Collaborative Team on Video Coding (JCT-VC) ofITU-T SG16 WP3 and ISO/IEC JTC1/SC29/WG1, Geneva, CH, April 17-May 7,2012 (“HEVC Draft 7”), B. Bross, et al., “High Efficiency Video Coding(HEVC) Text Specification Draft 8,” JCTVC-J1003_d7, Joint CollaborativeTeam on Video Coding (JCT-VC) of ITU-T SG16 WP3 and ISO/IECJTC1/SC29/WG1, Stockholm, SE, Jul. 11-20, 2012 (“HEVC Draft 8”), and B.Bross, et al., “High Efficiency Video Coding (HEVC) Text SpecificationDraft 9,” JCTVC-K1003_v7, Joint Collaborative Team on Video Coding(JCT-VC) of ITU-T SG16 WP3 and ISO/IEC JTC1/SC29/WG1, Shanghai, CN, Oct.10-19, 2012 (“HEVC Draft 9”).

Some aspects of this disclosure have been presented to the JCT-VC in M.Zhou, “AHG7: A Combined Study on JCTVC-I0216 and JCTVC-I0107,”JCTVC-I0425, Joint Collaborative Team on Video Coding (JCT-VC) of ITU-TSG16 WP3 and ISO/IEC JTC1/SC29/WG1, Geneva, Switzerland, Apr. 27,2012—May 7, 2012, and M. Zhou, “AHG7: Disallow Bi-Predictive Mode for8×4 and 4×8 Inter PUs,” JCTVC-J0086, Joint Collaborative Team on VideoCoding (JCT-VC) of ITU-T SG16 WP3 and ISO/IEC JTC1/SC29/WG1, Stockholm,Sweden, Jul. 11-20, 2012, both of which are incorporated by referenceherein in their entirety.

As previously discussed, motion compensation requires a significantamount of memory bandwidth, especially for smaller prediction unitsizes. In HEVC, inter-prediction of 4×4 PUs is permanently disabled.Thus, the worst case bandwidth requirements can occur when bi-predicted4×8 or 8×4 PUs are used. To reduce the memory bandwidth requirements forsuch smaller PUs, embodiments of the invention provide for restrictinginter-coded PUs of small block sizes to be coded only in auni-predictive mode, i.e., forward prediction or backward prediction.More specifically, PUs of specified restricted sizes in bi-predictedslices (B slices) are forced to be uni-predicted.

FIG. 2 shows a block diagram of a digital system that includes a sourcedigital system 200 that transmits encoded video sequences to adestination digital system 202 via a communication channel 216. Thesource digital system 200 includes a video capture component 204, avideo encoder component 206, and a transmitter component 208. The videocapture component 204 is configured to provide a video sequence to beencoded by the video encoder component 206. The video capture component204 may be, for example, a video camera, a video archive, or a videofeed from a video content provider. In some embodiments, the videocapture component 204 may generate computer graphics as the videosequence, or a combination of live video, archived video, and/orcomputer-generated video.

The video encoder component 206 receives a video sequence from the videocapture component 204 and encodes it for transmission by the transmittercomponent 208. The video encoder component 206 receives the videosequence from the video capture component 204 as a sequence of pictures,divides the pictures into largest coding units (LCUs), and encodes thevideo data in the LCUs. The video encoder component 206 may beconfigured to reduce memory compensation bandwidth by forcing PUs ofspecified restricted sizes in B slices to be uni-predicted during theencoding process as described herein. An embodiment of the video encodercomponent 206 is described in more detail herein in reference to FIG. 3.

The transmitter component 208 transmits the encoded video data to thedestination digital system 202 via the communication channel 216. Thecommunication channel 216 may be any communication medium, orcombination of communication media suitable for transmission of theencoded video sequence, such as, for example, wired or wirelesscommunication media, a local area network, or a wide area network.

The destination digital system 202 includes a receiver component 210, avideo decoder component 212 and a display component 214. The receivercomponent 210 receives the encoded video data from the source digitalsystem 200 via the communication channel 216 and provides the encodedvideo data to the video decoder component 212 for decoding. The videodecoder component 212 reverses the encoding process performed by thevideo encoder component 206 to reconstruct the LCUs of the videosequence. The video decoder component 212 may be configured to decodePUs of specified restricted sizes in B slices during the decodingprocess as described herein, thus reducing the motion compensationbandwidth consumed over the prior art. An embodiment of the videodecoder component 212 is described in more detail below in reference toFIG. 4.

The reconstructed video sequence is displayed on the display component214. The display component 214 may be any suitable display device suchas, for example, a plasma display, a liquid crystal display (LCD), alight emitting diode (LED) display, etc.

In some embodiments, the source digital system 200 may also include areceiver component and a video decoder component and/or the destinationdigital system 202 may include a transmitter component and a videoencoder component for transmission of video sequences both directionsfor video streaming, video broadcasting, and video telephony. Further,the video encoder component 206 and the video decoder component 212 mayperform encoding and decoding in accordance with one or more videocompression standards. The video encoder component 206 and the videodecoder component 212 may be implemented in any suitable combination ofsoftware, firmware, and hardware, such as, for example, one or moredigital signal processors (DSPs), microprocessors, discrete logic,application specific integrated circuits (ASICs), field-programmablegate arrays (FPGAs), etc.

FIG. 3 is a block diagram of the LCU processing portion of an examplevideo encoder. A coding control component (not shown) sequences thevarious operations of the LCU processing, i.e., the coding controlcomponent runs the main control loop for video encoding. The codingcontrol component receives a digital video sequence and performs anyprocessing on the input video sequence that is to be done at the picturelevel, such as determining the coding type (I, P, or B) of a picturebased on the high level coding structure, e.g., IPPP, IBBP,hierarchical-B, and dividing a picture into LCUs for further processing.

In addition, for pipelined architectures in which multiple LCUs may beprocessed concurrently in different components of the LCU processing,the coding control component controls the processing of the LCUs byvarious components of the LCU processing in a pipeline fashion. Forexample, in many embedded systems supporting video processing, there maybe one master processor and one or more slave processing modules, e.g.,hardware accelerators. The master processor operates as the codingcontrol component and runs the main control loop for video encoding, andthe slave processing modules are employed to off load certaincompute-intensive tasks of video encoding such as motion estimation,motion compensation, intra prediction mode estimation, transformationand quantization, entropy coding, and loop filtering. The slaveprocessing modules are controlled in a pipeline fashion by the masterprocessor such that the slave processing modules operate on differentLCUs of a picture at any given time. That is, the slave processingmodules are executed in parallel, each processing its respective LCUwhile data movement from one processor to another is serial.

The LCU processing receives LCUs 300 of the input video sequence fromthe coding control component and encodes the LCUs 300 under the controlof the coding control component to generate the compressed video stream.The LCUs 300 in each picture are processed in row order. The LCUs 300from the coding control component are provided as one input of a motionestimation component (ME) 320, as one input of an intra-predictionestimation component (IPE) 324, and to a positive input of a combiner302 (e.g., adder or subtractor or the like). Further, although notspecifically shown, the prediction mode of each picture as selected bythe coding control component is provided to a mode decision component328 and the entropy coding component 336.

The storage component 318 provides reference data to the motionestimation component 320 and to the motion compensation component 322.The reference data may include one or more previously encoded anddecoded pictures, i.e., reference pictures.

The motion estimation component 320 provides motion data information tothe motion compensation component 322 and the entropy coding component336. In general, the motion estimation component 320 performs tests onCUs in an LCU based on multiple inter-prediction modes (e.g., skip mode,merge mode, and normal or direct inter-prediction), PU sizes, and TUsizes using reference picture data from storage 318 to choose the bestCU partitioning, PU/TU partitioning, inter-prediction modes, motionvectors, etc. based on coding cost, e.g., a rate distortion coding cost.To perform the tests, the motion estimation component 320 may divide anLCU into CUs according to the maximum hierarchical depth of thequadtree, and divide each CU into PUs according to the unit sizes of theinter-prediction modes and into TUs according to the transform unitsizes, and calculate the coding costs for each PU size, prediction mode,and transform unit size for each CU. The motion estimation component 322may perform the method of FIG. 5 in which PUs of specified restrictedsizes in B slices are forced to be uni-predicted to determine the bestprediction mode for each PU. The motion estimation component 320provides the motion vector (MV) or vectors and the prediction mode foreach PU in the selected CU partitioning to the motion compensationcomponent (MC) 322.

The motion compensation component 322 receives the selectedinter-prediction mode and mode-related information from the motionestimation component 320 and generates the inter-predicted CUs. Theinter-predicted CUs are provided to the mode decision component 328along with the selected inter-prediction modes for the inter-predictedPUs and corresponding TU sizes for the selected CU/PU/TU partitioning.The coding costs of the inter-predicted CUs are also provided to themode decision component 328.

The intra-prediction estimation component 324 (IPE) performsintra-prediction estimation in which tests on CUs in an LCU based onmultiple intra-prediction modes, PU sizes, and TU sizes are performedusing reconstructed data from previously encoded neighboring CUs storedin a buffer (not shown) to choose the best CU partitioning, PU/TUpartitioning, and intra-prediction modes based on coding cost, e.g., arate distortion coding cost. To perform the tests, the intra-predictionestimation component 324 may divide an LCU into CUs according to themaximum hierarchical depth of the quadtree, and divide each CU into PUsaccording to the unit sizes of the intra-prediction modes and into TUsaccording to the transform unit sizes, and calculate the coding costsfor each PU size, prediction mode, and transform unit size for each PU.The intra-prediction estimation component 324 provides the selectedintra-prediction modes for the PUs, and the corresponding TU sizes forthe selected CU partitioning to the intra-prediction component (IP) 326.The coding costs of the intra-predicted CUs are also provided to theintra-prediction component 326.

The intra-prediction component 326 (IP) receives intra-predictioninformation, e.g., the selected mode or modes for the PU(s), the PUsize, etc., from the intra-prediction estimation component 324 andgenerates the intra-predicted CUs. The intra-predicted CUs are providedto the mode decision component 328 along with the selectedintra-prediction modes for the intra-predicted PUs and corresponding TUsizes for the selected CU/PU/TU partitioning. The coding costs of theintra-predicted CUs are also provided to the mode decision component328.

The mode decision component 328 selects between intra-prediction of a CUand inter-prediction of a CU based on the intra-prediction coding costof the CU from the intra-prediction component 326, the inter-predictioncoding cost of the CU from the motion compensation component 322, andthe picture prediction mode provided by the coding control component.Based on the decision as to whether a CU is to be intra- or inter-coded,the intra-predicted PUs or inter-predicted PUs are selected. Theselected CU/PU/TU partitioning with corresponding modes and other moderelated prediction data (if any) such as motion vector(s) and referencepicture index (indices), are provided to the entropy coding component336.

The output of the mode decision component 328, i.e., the predicted PUs,is provided to a negative input of the combiner 302 and to the combiner316. The associated transform unit size is also provided to thetransform component 304. The combiner 302 subtracts a predicted PU fromthe original PU. Each resulting residual PU is a set of pixel differencevalues that quantify differences between pixel values of the original PUand the predicted PU. The residual blocks of all the PUs of a CU form aresidual CU for further processing.

The transform component 304 performs block transforms on the residualCUs to convert the residual pixel values to transform coefficients andprovides the transform coefficients to a quantize component 306. Morespecifically, the transform component 304 receives the transform unitsizes for the residual CU and applies transforms of the specified sizesto the CU to generate transform coefficients. Further, the quantizecomponent 306 quantizes the transform coefficients based on quantizationparameters (QPs) and quantization matrices provided by the codingcontrol component and the transform sizes and provides the quantizedtransform coefficients to the entropy coding component 336 for coding inthe bit stream.

The entropy coding component 336 entropy encodes the relevant data,i.e., syntax elements, output by the various encoding components and thecoding control component using context-adaptive binary arithmetic coding(CABAC) to generate the compressed video bit stream. Among the syntaxelements that are encoded are picture parameter sets, flags indicatingthe CU/PU/TU partitioning of an LCU, the prediction modes (inter orintra) for the CUs, the prediction modes of PUs within CUs (normal interprediction mode, merge mode, or intra-prediction mode), and thequantized transform coefficients for the CUs. For PUs predicted in mergemode, a syntax element for an index identifying a particular mergingcandidate in a merging candidate list may be encoded. For PUs predictedin normal prediction mode, a syntax element for an index identifying aparticular motion vector predictor (MVP) candidate in an MVP candidatelist may be encoded along with syntax elements for motion vectordifferences (MVDS), prediction direction, and a reference picture index(or indices). Merging candidate lists and MVP candidate lists areexplained in reference to FIG. 5 herein. The entropy coding component336 also codes relevant data from the in-loop filters (described below).

The LCU processing includes an embedded decoder. As any compliantdecoder is expected to reconstruct an image from a compressed bitstream, the embedded decoder provides the same utility to the videoencoder. Knowledge of the reconstructed input allows the video encoderto transmit the appropriate residual energy to compose subsequentpictures.

The quantized transform coefficients for each CU are provided to aninverse quantize component (IQ) 312, which outputs a reconstructedversion of the transform result from the transform component 304. Thedequantized transform coefficients are provided to the inverse transformcomponent (IDCT) 314, which outputs estimated residual informationrepresenting a reconstructed version of a residual CU. The inversetransform component 314 receives the transform unit size used togenerate the transform coefficients and applies inverse transform(s) ofthe specified size to the transform coefficients to reconstruct theresidual values. The reconstructed residual CU is provided to thecombiner 316.

The combiner 316 adds the original predicted CU to the residual CU togenerate a reconstructed CU, which becomes part of reconstructed picturedata. The reconstructed picture data is stored in a buffer (not shown)for use by the intra-prediction estimation component 324.

Various in-loop filters may be applied to the reconstructed picture datato improve the quality of the reference picture data used forencoding/decoding of subsequent pictures. The in-loop filters mayinclude a deblocking filter component 330, a sample adaptive offsetfilter (SAO) component 332, and an adaptive loop filter (ALF) component334. The in-loop filters 330, 332, 334 are applied to each reconstructedLCU in the picture and the final filtered reference picture data isprovided to the storage component 318. In some embodiments, the ALFcomponent 334 may not be present.

FIG. 4 is a block diagram of an example video decoder. The video decoderoperates to reverse the encoding operations, i.e., entropy coding,quantization, transformation, and prediction, performed by the videoencoder of FIG. 3 to regenerate the pictures of the original videosequence. In view of the above description of a video encoder, one ofordinary skill in the art will understand the functionality ofcomponents of the video decoder without detailed explanation.

The entropy decoding component 400 receives an entropy encoded(compressed) video bit stream and reverses the entropy encoding usingCABAC decoding to recover the encoded syntax elements, e.g., CU, PU, andTU structures of LCUs, quantized transform coefficients for CUs, motionvectors, prediction modes, ALF coefficients, etc. The decoded syntaxelements are passed to the various components of the decoder as needed.For example, decoded prediction modes are provided to theintra-prediction component (IP) 414 or motion compensation component(MC) 410. If the decoded prediction mode is an inter-prediction mode,the entropy decoding component 400 determines the motion vector(s),reference picture indices, etc. as needed for PUs and provides themotion vector(s) to the motion compensation component 410. The entropydecoding component may perform the method of FIG. 6 to determine motionvectors, etc., for an inter-predicted PU.

The inverse quantize component (IQ) 402 de-quantizes the quantizedtransform coefficients of the CUs. The inverse transform component 404transforms the frequency domain data from the inverse quantize component402 back to the residual CUs. That is, the inverse transform component404 applies an inverse unit transform, i.e., the inverse of the unittransform used for encoding, to the de-quantized residual coefficientsto produce reconstructed residual values of the CUs.

A residual CU supplies one input of the addition component 406. Theother input of the addition component 406 comes from the mode switch408. When an inter-prediction mode is signaled in the encoded videostream, the mode switch 408 selects predicted PUs from the motioncompensation component 410 and when an intra-prediction mode issignaled, the mode switch selects predicted PUs from theintra-prediction component 414.

The motion compensation component 410 receives reference data from thestorage component 412 and applies the motion compensation computed bythe encoder and transmitted in the encoded video bit stream to thereference data to generate a predicted PU. That is, the motioncompensation component 410 uses the motion vector(s), reference index(or indices), etc. from the entropy decoder 400 and the reference datato generate a predicted PU.

The intra-prediction component 414 receives reconstructed samples frompreviously reconstructed PUs of a current picture from the storagecomponent 412 and performs the intra-prediction computed by the encoderas signaled by an intra-prediction mode transmitted in the encoded videobit stream using the reconstructed samples as needed to generate apredicted PU.

The addition component 406 generates a reconstructed CU by adding thepredicted PUs selected by the mode switch 408 and the residual CU. Theoutput of the addition component 406, i.e., the reconstructed CUs, isstored in the storage component 412 for use by the intra-predictioncomponent 414.

In-loop filters may be applied to reconstructed picture data to improvethe quality of the decoded pictures and the quality of the referencepicture data used for decoding of subsequent pictures. The appliedin-loop filters are the same as those of the encoder, i.e., a deblockingfilter 416, a sample adaptive offset filter (SAO) 418, and an adaptiveloop filter (ALF) 420. In some embodiments, the ALF component 420 maynot be present. The in-loop filters may be applied on an LCU-by-LCUbasis and the final filtered reference picture data is provided to thestorage component 412.

FIG. 5 is a flow diagram of a method for selecting a bestinter-prediction mode for a PU that forces PUs of selected restrictedsizes in B slices to be uni-predicted. The particular restricted PUsizes may be defined by the coding standard in use, e.g., HEVC. In someembodiments, the restricted PU sizes are PU sizes of 8×8 or smaller. Insome embodiments, the restricted PU sizes are 4×8 and 8×4.

As previously mentioned, multiple inter-prediction modes are consideredfor a PU during motion estimation, i.e., skip mode (if a PU is also aCU), merge mode, and normal inter-prediction mode. Merge mode isdesigned to reduce coding overhead by allowing an inter-predicted PU toinherit motion data, i.e., motion vectors, prediction direction, andreference picture indices, from a position selected from neighboringmotion data positions in the same picture and a temporal motion dataposition derived based on a co-located block of the same size as the PUin a reference picture, referred to as the co-located PU. FIG. 7illustrates the particular motion data positions considered for a PU asdefined in HEVC Draft 6 (and later drafts).

The skip mode is a special case of merge mode where the PU is also thecoding unit (CU) and the CU has all zero transform coefficients. Regular(normal) motion vector coding for inter-prediction of a PU considersmotion vectors of neighboring motion data positions in the same pictureand a temporal motion data position derived based on a co-located PU foruse as motion vector predictors for the PU. The same motion datapositions considered for merge/skip mode are considered for normalinter-prediction mode.

In the method of FIG. 5, a merging candidate list is constructed 500 forthe PU. The merging candidate list is formed by considering mergingcandidates from the seven motion data positions of FIG. 7. To derivemotion data from a motion data position, the motion data is copied fromthe corresponding PU which contains (or covers) the motion dataposition. The specific derivation of a merging candidate list from thesemotion data positions is described in HEVC draft 6 (and later drafts).If the PU is in a forward predicted slice (P slice), the mergingcandidates in the merging candidate list are either from forwardpredicted PUs or are zero motion vector merging candidates configuredfor forward prediction. As is explained in HEVC Draft 6 (and laterdrafts), zero merging candidates are added to a merging candidate listwhen the merging candidate list derived from considering the candidatemotion data positions contains fewer than a specified number of mergingcandidates.

If the PU is in a B slice, the merging candidates in the mergingcandidate list may be from forward predicted PUs, backward predictedPUs, bi-predicted PUs, or combined bi-predictive merging candidates, ormay be zero motion vector merging candidates configured forbi-prediction. A combined bi-predictive merging candidate is generatedby combining the merging candidates in the merging candidate list in apre-defined combination priority order specified by HEVC. As with zeromerging candidates, combined bi-predictive merging candidates may beadded to a merging candidate list when the merging candidate listderived from considering the candidate motion data positions containsless than a specified number of merging candidates.

A merging candidate includes motion vector information, prediction flaginformation, and reference picture index information for a candidatemotion data position. A merging candidate may include sufficient entriesto accommodate a bi-directionally predicted PU, i.e., entries for aforward motion vector, a backward motion vector, a forward referencepicture index, a backward reference picture index, and a prediction flagindicating prediction direction, i.e., forward, backward, orbi-directional. The prediction flag may be composed of two predictionlist utilization flags used to indicate which of two reference picturelists, a forward reference picture list and a backward reference picturelist, is to be used. The forward reference picture list may be referredto as list0 or 10 and the backward reference picture list may bereferred to as list1 or 11. Each reference picture index is an indexinto a respective one of the reference picture lists. For a motion dataposition covered by a forward predicted PU, the merging candidateentries for the prediction flag, the forward motion vector, and theforward reference picture index will be valid and the remaining entriesmay have placeholder values. For a motion data position covered by abackward predicted PU, the merging candidate entries for the predictionflag, the backward motion vector, and the backward reference pictureindex will be valid and the remaining entries may have placeholdervalues. For a motion data position covered by a bi-directionallypredicted PU, all merging candidate entries may be valid.

After construction 500 of the merging candidate list for the PU, if thePU is in a P slice (502—YES), a list0 motion vector predictor (MVP)candidate list is constructed 504 for the PU. A motion vector predictorcandidate list is referred to as an advanced motion vector predictor(AMVP) candidate list in HEVC. The list0 MVP candidate list is formed byconsidering MVP candidates from the seven motion data positions of FIG.7. The specific derivation of a list0 MVP candidate list from thesemotion data positions is described in HEVC draft 6 (and later drafts).In general, an MVP candidate for a motion data position is the motionvectors from the motion data of the corresponding PU which contains(covers) the motion data position. If the MVP candidate list resultingfrom considering the motion data positions contains less than aspecified number of MVP candidates, one or more zero MVP candidates maybe added to the list.

The merging candidate list and the list0 MVP candidate list are thenused to determine 514 the best inter-prediction mode for the PU. Thisdetermination may be performed as follows. Coding costs are computed foreach entry in the merging candidate list and, and the merging candidatewith the best result selected. Coding costs are also computed forcandidate motion vectors of normal inter-prediction mode with the bestresult selected. For coding cost computation of a forward-predictedcandidate motion vector of normative inter-prediction mode, motionvector costs relative to each entry in the list0 MVP candidate list maybe evaluated with the least motion vector cost added to the coding costof the forward-predicted candidate. The coding costs of using theselected merging candidate and the best candidate motion vector ofnormal inter-prediction mode are compared to decide whether merge modeor normal inter-prediction mode is the best inter-prediction mode forthe PU.

If the PU is not in a P-slice (502—NO), then it is in a B-slice. If thesize of the PU is a restricted size (506—YES), then a list0 MVPcandidate list 510 is constructed for the PU. In addition, bi-predictivemerging candidates (if any) in the merging candidate list are converted512 to uni-predictive merging candidates. As previously discussed,merging candidates for a PU in a B-slice may be forward, backward, orbi-predictive. Note that a full merging candidate list that may includebi-predictive merging candidates is constructed for the PU before anybi-predictive merging candidates are converted to uni-predictive.

In some embodiments, the bi-predictive merging candidates are convertedto list0 (forward predicted) merging candidates. This conversion may beaccomplished, for example, by invalidating the list1 motion data in abi-predictive merging candidate and setting the prediction flag toindicate forward (list0) prediction. In some embodiments, thebi-predictive merging candidates are converted to list1 (backwardpredicted) merging candidates. This conversion may be accomplished, forexample, by invalidating the list0 motion data in a bi-predictivemerging candidate and setting the prediction flag to indicate backward(list1) prediction. In some embodiments, each bi-predictive mergingcandidate is converted to either a list0 or list1 merging candidatebased on the values of the reference picture indices in thebi-predictive merging candidate. If the value of the forward (list0)reference picture index is less than or equal to the value of thebackward (list1) reference picture index, the bi-predictive mergingcandidate is converted to a list0 merging candidate by invalidating thelist1 motion data and setting the prediction flag to indicate forward(list0) prediction. Otherwise, the bi-predictive merging candidate isconverted to a list1 merging candidate by invalidating the list0 motiondata and setting the prediction flag to indicate backward (list1)prediction.

The modified merging candidate list and the list0 MVP candidate list arethen used to determine 514 the best inter-prediction mode for the PU.This determination may be performed as previously described.

If the PU is in a B-slice (502—NO) and the size is not a restricted size(506—NO), then list0 and list1 MVP candidate lists are constructed forthe PU 508. The list0 MVP candidate list and the list1 MVP candidatelist are formed by considering MVP candidates from the seven motion datapositions of FIG. 7. Note that the temporally co-located motion datapositions for the list0 MVP candidate list derivation will be in a list0(forward) reference picture and the temporally co-located motion datapositions for the list1 MVP candidate list derivation will be in a list1(backward) reference picture. The specific derivation of list0 and list1MVP candidate lists from these motion data positions is described inHEVC draft 6 (and later drafts). In general, an MVP candidate for amotion data position is the motion vectors from the motion data of thecorresponding PU which contains (covers) the motion data position. Ifthe MVP candidate list resulting from considering the motion datapositions contains less than a specified number of MVP candidates, oneor more zero MVP candidates may be added to the list.

The merging candidate list and the list0 and list1 MVP candidate listsare then used to determine 514 the best inter-prediction mode for thePU. This determination may be performed as follows. Coding costs arecomputed for each entry in the merging candidate list and, and themerging candidate with the best result selected. Coding costs are alsocomputed for candidate motion vectors of normal inter-prediction modewith the best result selected. For coding cost computation of aforward-predicted candidate motion vector of normal inter-predictionmode, motion vector costs relative to each entry in the list0 MVPcandidate list may be evaluated with the least motion vector cost addedto the coding cost of the forward-predicted candidate. For coding costcomputation of a backward-predicted candidate motion vector of normalinter-prediction mode, motion vector costs relative to each entry in thelist1 MVP candidate list may be evaluated with the least motion vectorcost added to the coding cost of the backward-predicted candidate. Forcoding cost computation of a bi-predictive candidate motion vector ofnormal inter-prediction mode, motion vector costs relative to each entryin the list0 and list1 MVP candidate list may be evaluated with theleast motion vector cost added to the coding cost of the bi-predictivecandidate. The coding cost of using the selected merging candidate andthe coding cost of the selected candidate motion vector of normalinter-prediction mode are compared to decide whether merge mode ornormal inter-prediction mode is the best inter-prediction mode for thePU.

As previously mentioned, if merge mode or normal inter-prediction modeis selected for a PU, the selected mode is signaled in the compressedbit stream. The prediction direction for the PU may also be signaledwhen the PU is in a B-slice. In some embodiments, the binarization ofthe prediction direction syntax elements for PUs of the restricted sizemay be different from that for PUs that are not of the restricted size.For PUs that are not of the restricted size, the prediction directionmay be binarized for CABAC encoding as shown in Table 1. For PUs of arestricted size, the prediction direction may be binarized for CABACencoding as shown in Table 2 as bi-prediction for such PUs is notallowed.

TABLE 1 Prediction direction CABAC binarization code word List 0 pluslist 1 bi-prediction 1 List 1 uni-prediction 01 List 0 uni-prediction 00

TABLE 2 Prediction direction CABAC binarization code word List 1uni-prediction 1 List 0 uni-prediction 0

FIG. 6 is a flow diagram of a method for decoding an inter-predicted PUwhen PUs of selected restricted sizes in B-slices are forced to beuni-predicted by the encoder as in the method of FIG. 5. If aninter-predicted PU to be decoded is predicted in merge or skip mode(600 - YES), a merging candidate list is constructed 602 for the PU.Construction of a merging candidate list in a decoder is the same asthat in an encoder and is previously described herein.

If the size of the PU is a restricted size (604—YES), bi-predictivemerging candidates in the merging candidate list (if any) are converted606 to uni-predictive merging candidates. Otherwise (604—NO), themerging candidate list is not changed. Note that a full mergingcandidate list that may include bi-predictive merging candidates if thePU is in a B-slice is constructed for the PU before any bi-predictivemerging candidates are converted to uni-predictive.

A merging candidate is selected 614 from the merging candidate listbased on the index signaled in the compressed bit stream, and theselected merging candidate is used to determine 616 motion vectors andother motion data for the PU. In this instance, the motion vectors ofthe merging candidate are used as the motion vectors for the PU. Inaddition, the other motion data of the merging candidate (predictiondirection and reference picture index (or indices)) is used for the PU.

The conversion of a bi-predictive merging candidate to a uni-predictivemerging candidate in a decoder is the same as that used in the encoder.In some embodiments, the bi-predictive merging candidates are convertedto list0 (forward predicted) merging candidates. This conversion may beaccomplished, for example, by invalidating the list1 motion data in abi-predictive merging candidate and setting the prediction flag toindicate forward (list0) prediction. In some embodiments, thebi-predictive merging candidates are converted to list1 (backwardpredicted) merging candidates. This conversion may be accomplished, forexample, by invalidating the list0 motion data in a bi-predictivemerging candidate and setting the prediction flag to indicate backward(list1) prediction. In some embodiments, each bi-predictive mergingcandidate is converted to either a list0 or list1 merging candidatebased on the values of the reference picture indices in thebi-predictive merging candidate. If the value of the forward (list0)reference picture index is less than or equal to the value of thebackward (list1) reference picture index, the bi-predictive mergingcandidate is converted to a list0 merging candidate by invalidating thelist1 motion data and setting the prediction flag to indicate forward(list0) prediction. Otherwise, the bi-predictive merging candidate isconverted to a list1 merging candidate by invalidating the list0 motiondata and setting the prediction flag to indicate backward (list1)prediction.

If an inter-predicted PU to be decoded is not predicted in merge or skipmode (600—NO), then if the PU is in a P-slice (608—YES), a list0 MVPcandidate list is constructed for the PU 610. Construction of a list0MVP candidate list in a decoder is the same as that in an encoder and ispreviously described herein. An MVP candidate is selected 614 from thelist0 MVP candidate list based on the index signaled in the compressedbit stream, and the selected MVP candidate is used to determine 616motion vectors for the PU. In this instance, the motion vector of theMVP candidate is added to the motion vector differences signaled in thecompressed bit stream to reconstruct the motion vector for the PU. Theadditional motion data (prediction direction and reference pictureindex) for the PU is decoded from the compressed bit stream.

If an inter-predicted PU to be decoded is not predicted in merge or skipmode (600—NO), and is not in a P-slice (608—NO), the PU is in a B-slice.A list0 MVP candidate list and/or a list1 MVP candidate list areconstructed for the PU depending on the particular prediction directionsignaled for the PU 612. Construction of a list0 or list1 MVP candidatelist in a decoder is the same as that in an encoder and is previouslydescribed herein. If the current PU is a forward-predicted PU, a list0MVP candidate list is constructed and an MVP candidate is selected 614from the list0 MVP candidate list based on the index for the listsignaled in the compressed bit stream. The selected MVP candidate isused to determine 616 the motion vector for the PU. If the current PU isa backward-predicted PU, a list1 MVP candidate list is constructed andan MVP candidate is selected 614 from the list1 MVP candidate list basedon the index for the list signaled in the compressed bit stream. Theselected MVP candidate is used to determine 616 the motion vector forthe PU.

If the current PU is a bi-predictive PU, a list0 MVP candidate list anda list1 MVP candidate list are constructed. An MVP candidate is selected614 from the list0 MVP candidate list based on the index for the listsignaled in the compressed bit stream. The selected MVP candidate isused to determine 616 the list0 motion vector for the PU. Furthermore,an MVP candidate is selected 614 from the list1 MVP candidate list basedon the index for the list signaled in the compressed bit stream. Theselected MVP candidate is used to determine 616 the list1 motion vectorfor the PU. The additional motion data (prediction direction andreference picture index (or indices)) for the PU is decoded from thecompressed bit stream.

Embodiments of the methods, encoders, and decoders described herein maybe implemented for virtually any type of digital system (e.g., a desktop computer, a laptop computer, a tablet computing device, a netbookcomputer, a handheld device such as a mobile (i.e., cellular) phone, apersonal digital assistant, a digital camera, etc.). FIG. 8 is a blockdiagram of an example digital system suitable for use as an embeddedsystem that may be configured to an embodiment of the method of FIG. 5during encoding of a video stream and/or an embodiment of the method ofFIG. 6 during decoding of an encoded video bit stream. This examplesystem-on-a-chip (SoC) is representative of one of a family of DaVinciTMDigital Media Processors, available from Texas Instruments, Inc. ThisSoC is described in more detail in “TMS320DM6467 Digital MediaSystem-on-Chip”, SPRS403G, December 2007 or later, which is incorporatedby reference herein.

The SoC 800 is a programmable platform designed to meet the processingneeds of applications such as video encode/decode/transcode/transrate,video surveillance, video conferencing, set-top box, medical imaging,media server, gaming, digital signage, etc. The SoC 800 provides supportfor multiple operating systems, multiple user interfaces, and highprocessing performance through the flexibility of a fully integratedmixed processor solution. The device combines multiple processing coreswith shared memory for programmable video and audio processing with ahighly-integrated peripheral set on common integrated substrate.

The dual-core architecture of the SoC 800 provides benefits of both DSPand Reduced Instruction Set Computer (RISC) technologies, incorporatinga DSP core and an ARM926EJ-S core. The ARM926EJ-S is a 32-bit RISCprocessor core that performs 32-bit or 16-bit instructions and processes32-bit, 16-bit, or 8-bit data. The DSP core is a TMS320C64x+™ core witha very-long-instruction-word (VLIW) architecture. In general, the ARM isresponsible for configuration and control of the SoC 800, including theDSP Subsystem, the video data conversion engine (VDCE), and a majorityof the peripherals and external memories. The switched central resource(SCR) is an interconnect system that provides low-latency connectivitybetween master peripherals and slave peripherals. The SCR is thedecoding, routing, and arbitration logic that enables the connectionbetween multiple masters and slaves that are connected to it.

The SoC 800 also includes application-specific hardware logic, on-chipmemory, and additional on-chip peripherals. The peripheral set includes:a configurable video port (Video Port I/F), an Ethernet MAC (EMAC) witha Management Data Input/Output (MDIO) module, a 4-bit transfer/4-bitreceive VLYNQ interface, an inter-integrated circuit (I2C) businterface, multichannel audio serial ports (McASP), general-purposetimers, a watchdog timer, a configurable host port interface (HPI);general-purpose input/output (GPIO) with programmable interrupt/eventgeneration modes, multiplexed with other peripherals, UART interfaceswith modem interface signals, pulse width modulators (PWM), an ATAinterface, a peripheral component interface (PCI), and external memoryinterfaces (EMIFA, DDR2). The video port I/F is a receiver andtransmitter of video data with two input channels and two outputchannels that may be configured for standard definition television(SDTV) video data, high definition television (HDTV) video data, and rawvideo data capture.

As shown in FIG. 8, the SoC 800 includes two high-definitionvideo/imaging coprocessors (HDVICP) and a video data conversion engine(VDCE) to offload many video and image processing tasks from the DSPcore. The VDCE supports video frame resizing, anti-aliasing, chrominancesignal format conversion, edge padding, color blending, etc. The HDVICPcoprocessors are designed to perform computational operations requiredfor video encoding such as motion estimation, motion compensation,intra-prediction, transformation, quantization, and in-loop filtering.Further, the distinct circuitry in the HDVICP coprocessors that may beused for specific computation operations is designed to operate in apipeline fashion under the control of the ARM subsystem and/or the DSPsubsystem.

As was previously mentioned, the SoC 800 may be configured to perform anembodiment of the method of FIG. 5 during encoding of a video streamand/or an embodiment of the method of FIG. 6 during decoding of anencoded video bit stream. For example, the coding control of the videoencoder of FIG. 3 may be executed on the DSP subsystem or the ARMsubsystem and at least some of the computational operations of the blockprocessing, including the intra-prediction and inter-prediction modeselection in which an embodiment of the method of FIG. 5 is used,transformation, quantization, and entropy encoding may be executed onthe HDVICP coprocessors. Similarly, at least some of the computationaloperations of the various components of the video decoder of FIG. 4,including entropy decoding in which an embodiment of the method of FIG.6 is used, inverse quantization, inverse transformation,intra-prediction, and motion compensation may be executed on the HDVICPcoprocessors.

Other Embodiments

While the invention has been described with respect to a limited numberof embodiments, those skilled in the art, having benefit of thisdisclosure, will appreciate that other embodiments can be devised whichdo not depart from the scope of the invention as disclosed herein.

For example, embodiments of the inventions are described herein assumingthe motion data positions of FIG. 7 are used for derivation of a mergingcandidate list, a list0 MVP list, and a list1 MVP lists, and thederivation (construction) of each list is performed as described in HEVCDraft 6. One of ordinary skill in the art will understand embodiments inwhich the motion data positions may be different and/or the listderivations may be different.

Embodiments of the methods, encoders, and decoders described herein maybe implemented in hardware, software, firmware, or any combinationthereof. If completely or partially implemented in software, thesoftware may be executed in one or more processors, such as amicroprocessor, application specific integrated circuit (ASIC), fieldprogrammable gate array (FPGA), or digital signal processor (DSP). Thesoftware instructions may be initially stored in a computer-readablemedium and loaded and executed in the processor. In some cases, thesoftware instructions may also be sold in a computer program product,which includes the computer-readable medium and packaging materials forthe computer-readable medium. In some cases, the software instructionsmay be distributed via removable computer readable media, via atransmission path from computer readable media on another digitalsystem, etc. Examples of computer-readable media include non-writablestorage media such as read-only memory devices, writable storage mediasuch as disks, flash memory, memory, or a combination thereof.

It is therefore contemplated that the appended claims will cover anysuch modifications of the embodiments as fall within the true scope ofthe invention.

What is claimed is:
 1. A method comprising: constructing a mergingcandidate list for a prediction unit (PU) of a picture; determining thatthe PU is in a bi-predicted slice of the picture; and converting abi-predicted merging candidate in the merging candidate list to aforward predicted merging candidate when a forward reference pictureindex for the bi-predicted merging candidate is less than or equal to abackward reference picture index for the bi-predicted merging candidate.2. The method of claim 1, wherein the size of the PU is either 8×4 and4×8.
 3. The method of claim 1, wherein determining that the PU is in abi-predicted slice comprises decoding an indicator that a slicecomprising the PU is bi-predicted.
 4. The method of claim 1, furthercomprising: encoding a prediction direction of the PU in a compressedbit stream.
 5. The method of claim 1, further comprising determining aninter-prediction mode for the PU using the merging candidate list.
 6. Anon-transitory computer readable medium storing software instructionsthat, when executed by a processor, cause a method to be performed, themethod comprising: constructing a merging candidate list for aprediction unit (PU) of a picture; determining that the PU is in abi-predicted slice of the picture; and converting a bi-predicted mergingcandidate in the merging candidate list to a forward predicted mergingcandidate when a forward reference picture index for the bi-predictedmerging candidate is less than or equal to a backward reference pictureindex for the bi-predicted merging candidate.
 7. The non-transitorycomputer readable medium of claim 6, wherein the size of the PU iseither 8×4 and 4×8.
 8. The non-transitory computer readable medium ofclaim 6, wherein the method further comprises: encoding a predictiondirection of the PU in a compressed bit stream.
 9. The non-transitorycomputer readable medium of claim 6, wherein the method furthercomprises determining an inter-prediction mode for the PU using themerging candidate list.